1. Field of the Invention
The invention relates to global positioning system methods, apparatus and signals, particularly employing corrections enabling performance suitable for code-phase-differential and real-time-kinematic applications.
2. Prior Art
Use of the global positioning system (GPS) to determine position has become commonplace, though the position determination has limitations. A GPS position fix can have an error which arises from a variety of unintended sources and, until recently, from the effect of Selective Availability. These errors can vary over time and with location. Various techniques have been developed to reduce the errors.
One approach is basic Differential GPS (DGPS), in which a fixed reference station at a known location generates corrections for all satellites in view. The corrections are transmitted to a roving GPS receiver, where they are used to remove common-mode errors, i.e., errors common to the reference station and the roving receiver. Residual errors increase as the distance between the roving receiver and the reference station increases, a phenomenon called geographic de-correlation. The amount of degradation of accuracy with distance from the fixed reference station depends on the state of the ionosphere, the troposphere, and errors in calculating satellite position, and can be as much as 1 meter for every 100 km (60 miles).
Another approach is network DGPS, in which four or more reference stations each collect data for all satellites in view at their respective locations. The reference-station data are transmitted to a network central processor which uses the data to determine the errors for each satellite in view. These errors are transmitted to the roving DGPS receiver, which uses these errors to generate corrections applicable to the location of the roving receiver. This capability added to a roving receiver has been termed a “virtual reference station” (VRS) because the result is like having a reference station at the location of the roving receiver. The VRS technique uses the correction data to remove actual satellite errors. Residual errors are essentially constant.
The VRS technique has advantages over a fixed base station. The satellite differential correction signals give corrections which are valid over a wide area. These wide area differential correction signals are used by the VRS to compute a differential correction applicable to the location of the VRS, at any place within the satellite view area. The computed correction is the same as a base station would generate if it were at the user's location. This correction is constantly updated so the corrections remain accurate as the user moves around.
The Wide-Area Augmentation System (WAAS) being deployed in the United States also is intended to provide differential correction signals which are valid over a wide area. One feature of WAAS is that it employs a large Kalman filter to generate differential corrections from measurements taken at the reference stations. This filter addresses many parameters simultaneously, so that it requires more processing power than is desirable.
A limitation of the VRS technique and of WAAS is that the differential correction signals, while valid over a wide area, are still not sufficiently accurate for some purposes. There is a need for GPS methods and apparatus providing greater accuracy, and which can be implemented with modest processing resources such as personal computers.